Colorectal cancer is typically considered a disease of aging — most new cases are diagnosed in people over age 50. But even as the rates decrease in older adults, scientists have documented a worrisome trend in the opposite direction among patients in their 20s and 30s. Now, data from national cancer registries in Canada add to the evidence that colorectal cancer rates are rising in younger adults. The increases may even be accelerating.

“We thought that this trend would slow down or level off after people first noticed it a few years ago,” said Darren Brenner, a molecular cancer epidemiologist at the University of Calgary and lead author of the new study, published on Wednesday in the journal JAMA Network Open. “But every year we keep seeing the increase in colorectal cancer among young people, and it is very alarming.” Between 2006 and 2015, the last year for which figures are available, colorectal cancer rates increased by 3.47 percent among Canadian men under age 50, Dr. Brenner and his colleagues found. And from 2010 to 2015, rates increased by 4.45 percent among women under age 50. Yet colon and rectal cancers have been steadily decreasing among older adults in Canada because of increased awareness of the disease and widespread use of screening tests like colonoscopies, which can identify and remove colon polyps before cancer develops.

The pattern is quite similar to that observed by researchers in the United States. Researchers at the University of Texas at Austin reported last week that the proportion of newly diagnosed colorectal patients under age 50 rose from 10 percent in 2004 to 12.2 percent in 2015. Younger patients were also likely to have advanced cases more often than older patients. Over all, the risk of colorectal cancer is still much lower in younger adults than in older ones. But the continuing uptick means that millennials will most likely carry an elevated risk as they get older.

“They’ll carry that risk with them, so that they have a much higher risk than their parents when they reach their 50s and 60s,” said Rebecca Siegel, an epidemiologist at the American Cancer Society. Recent lifestyle changes may be partly to blame. Obesity and sedentary lifestyles, for example, are linked to colorectal cancer, as are poor diets low in fiber. Patients with chronic inflammation or Type 2 diabetes have also been found to be at increased risk for the disease. But experts are not entirely convinced these are the only factors at work. Trends in obesity among people of different ethnic and racial backgrounds don’t always correspond to an increase in colorectal cancer, according to Ms. Siegel.

Some studies have found that obesity brings increased risk of colon cancer, while others, including the new JAMA research, have found a greater increase in cancers of the rectum. Until there is more research into what is causing the increase in colorectal cancers, Ms. Siegel encourages younger people to be more proactive about identifying signs early on. Persistent constipation, cramps, bloating, blood in stool, unexplained weight loss and fatigue can all be symptoms. Younger people and their doctors often overlook the warning signs because “cancer is not on their radar,” Ms. Siegel said.

The American Cancer Society now recommends screening average-risk individuals for colorectal cancer starting at age 45. Researchers in Canada also are considering changes to screening recommendations. But these revisions are unlikely to help prevent cancers among patients who are even younger. “We need to understand why this trend is occurring in young people in order to prevent it,” Dr. Brenner said.